The invention relates to novel modified polypropylene compositions, especially for automobile exterior applications. More particularly, the invention relates to modified polypropylene compositions comprising a heterophasic propylene copolymer, a further elastomeric copolymer and an inorganic filler. The modified composition has excellent properties, especially impact strength, stiffness, elasticity and surface stability.
The automotive market increasingly requires polypropylene compounds for car exterior applications having a performance which is not achievable with conventional compounding. One of the critical requirements for exterior applications in many cases is the high surface stability of the moulded parts which are painted such as bumper, body panels etc.
It is well known that the mechanical performance of polypropylene/rubber compounds is determined by the phase structure and thus composition morphology. With low molecular weight rubbers (reactor made and externally), the tensile elongation of the final filler compounds for typical exterior applications is improved. However, the low molecular weight rubber fraction causes unsatisfactory surface stability, which results in unsatisfactory paint adhesion.
The main reason for the low surface stability was identified to be in the surface structure of the moulded part. Studies (Moffitt et al; J. Pol. Sci. 40 (2002), 2842; Ryntz; Proc. Org. Coat. 27 (1996), 241)) have shown that when a polypropylene/rubber system is injection moulded, the surface morphology of formed parts, due to the shear induced crystallisation, could be described as a surface layer of a polypropylene rich region, beneath which a rubber (particles) rich layer lies.
Surface stability of such multiphase polymer systems (polypropylene/rubber) depends very strongly on the phase separation behaviour in the surface region. The role of interface management between polypropylene and rubber phases in this region e.g. control of miscibility between the components is a major factor affecting the ability of the moulded part to resist surface damage caused by external forces. Stresses applied to the polypropylene/rubber part in form of compressive shearing events, e.g. scratches, gouges etc. often result in cohesive debondment due to the week phase adhesion of the systems appearing in form of “ripping” of the surface region (10-100 μm). The cohesive failure is a result of improperly controlled phase bounding arising by incompatibility between two polymers (Ryntz, ACS Symp. Ser. 805 (2002), 266).
There exists a continuing need for new polypropylene compositions for injection moulding of automobile exterior parts, where the compositions or the injection moulded parts produced therefrom have to fulfill the following requirements simultaneously: High impact strength, high stiffness, high elasticity, high surface stability.